132 PHOTOSYNTHESIS 



action of light is to produce say, with some compound XY, 

 or with a complex of two compounds, X and Y the products 

 XH and YOH, both being regarded as radicals, analogous to 

 semiquinone compounds. We cannot yet identify either X 

 or Y in the case of chlorophyll; nor has it been shown that 

 X represents COg in some stage of reduction. Yet we can 

 see how this general scheme expresses the light process as 

 electron transfer and is compatible with the idea of hydro- 

 gen transfer in photosynthesis. Further there is no reason 

 why H and OH could not arise from water, a formulation 

 consistent with the general theory developed by van Niel 

 (Chapter 5). 



Such a formulation has been successfully applied by 

 Gaffron (1944) to the interpretation of his experiments with 

 Scenedesmus. In photoreduction the XH and YOH result 

 from the action of light; in the oxyhydrogen reaction the 

 YOH is derived in dark from the oxygen itself. 



Studies with the photochemistry of chlorophyll solutions 

 have not only shown the existence of photochemically 

 induced reversible states of the molecule but also under 

 appropriate conditions the formation of a reversibly oxidized 

 form of chlorophyll. Recently the existence of a reversibly 

 reduced form of chlorophyll (in pyridine as a solvent) has been 

 demonstrated. A reversible bleaching of bacteriochlorophyll 

 in vivo has also been demonstrated. However it has not been 

 established whether chlorophyll actually does undergo a 

 reversible chemical transformation which itself is an essen- 

 tial part of photosynthesis. 



Energy loss in hack reaction 



In the consideration of any possible detailed molecular 

 mechanism of the photochemical reaction it is necessary to 

 postulate some efficient means of preventing an immediate 

 dark reaction the reverse of that initiated by light. This is 

 because the process of photosynthesis is found to use the 

 light energy with high efficiency at low light intensities. 

 The chloroplasts and indeed the individual grana them- 

 selves have been shown to have a characteristic organized 

 structure. It may well be that this structure together with 



